8355230: Crash in fuzzer tests: assert(n != nullptr) failed: must not be null

[rwestrel]

During IGVN, TypeNode::make_paths_from_here_dead() follows data
nodes until a Phi. The Region input for the input that that logic
goes through to reach the Phi is null causing the crash. I propose
simply adding an extra check for that corner case.

---

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Issue

• JDK-8355230: Crash in fuzzer tests: assert(n != nullptr) failed: must not be null (Bug - P3)

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